Biodiesel is a renewable fuel which is produced from renewable sources such as vegetable oils, animal fats, and recycled cooking oil. It is used as an alternative biofuel in existing diesel engines without the need for engine modification. Biodiesels are biofuels consisting of long chain alkyl esters. It is synthesized by chemically reacting lipids such as vegetable oils and animal fats with an alcohol producing fatty acid esters. Biodiesel can be used singularly, or blended with petroleum based diesels in any proportions. They are also prevalently used as heating oil.